Simply add a field and use a script to both print your letter and update the field to show that it has been printed. I'd use a date or time stamp field and update it whith the current date or time stamp so that I can not only show that the letter was printed, but can show when it was printed.
If your button already performs a script when it prints the letter, all you'll need to do is define your field and then insert one of the following two steps into your print script:
Set Field [tablename::LetterPrintedTimeStampField; Get ( CurrentTimeStamp )]
Set Field [tablename::LetterPrintedDateField; Get ( CurrentDate )]
Thanks very much Phil.
Enjoy your weekend.